The conventional technique of this kind is disclosed in Japanese Patent Laid-Open publication No. 91982/1986. This Japanese Patent Laid-Open publication NO. 91982 discloses a discharge exciting excimer laser device in which for extending the life of laser gas, alumina ceramics inert to laser gas is used in place of quartz heretofore used to constitute a dielectric.
In the discharge exciting excimer laser device of this kind, it is necessary for increasing a laser output to activate preliminary ionization, when auxiliary discharge takes place between a main discharge electrode and an auxiliary discharge electrode, that is, photo ionization effect caused by ultraviolet luminescence from the auxiliary electrode and the supply of electron to a main discharge space.
To this end, it was necessary (1) to quicken a rise of voltage between the main discharge electrode and the auxiliary discharge electrode, (2) to increase a dielectric constant of an dielectric, (3) to reduce the sectional thickness of the dielectric, and (4) to improve a contact between the dielectric and the electrode. In addition, there was a technical problem of improving the insulating performance between the electrodes.
However, in the conventional techniques including the aforesaid publication, sufficient consideration has not been paid to the aforesaid points. Particularly, the shape of the dielectric in the aforesaid points (3) and (4) has been ignored.
More particularly, in the prior art, the dielectric to surround the auxiliary electrode was in the shape of cylinder (a round pipe) or flat plate, and the inner surface thereof in contact with the auxiliary electrode was curved, there were difficulties in that polishing work is difficult, and the operation for receiving the auxiliary electrode therein is cumbersome.
When a high voltage of the order of 30 kv was applied in order to obtain a high output, a dielectric breakdown possibly occurs in the dielectric construction having the aforementioned shape.
On the other hand, Japanese Patent Publication No. 64069/1988 discloses a discharge exciting pulse laser in which a dielectric in the form of a tube is used and a cooling medium is sealed into the tubular dielectric. The feature of this patent publication lies in that a dielectric is formed into a tubular configuration, into which is sealed a cooling medium, and nothing is described of the fact that a dielectric should be formed into a square pipe.